Liquid crystal display (LCD) devices have become increasingly important in displays which require very low consumption of electrical power or where the environment dictates a lightweight, planar, flat surface. Thus, LCD's are used in display devices such as wristwatches, pocket and personal computers, aircraft cockpit displays, etc.
In its simplest form, a field effect liquid crystal display device consists of a liquid crystal layer with opposite sides, a set of electrodes on either side of the liquid crystal layer and an alignment polymer layer between each set of electrodes and the liquid crystal layer. Alignment of the liquid crystal molecules occurs at a certain angle, referred to as the tilt angle, with respect to the plane of the inside of two glass plates. The inside of the glass plates is coated with sets of transparent electrodes (electrical conductors), usually indium-tin oxide (ITO). The sets of electrodes are etched into a pattern compatible with the information to be displayed by the LCD. The alignment process is most easily carried out by solution casting (spin coating, roller coating, dipping, spraying, printing and/or doctor blading) an organic polymer onto the two glass/ITO substrates. After removal of solvents and/or curing of the polymer layers, the glass plates are usually rubbed or buffed in one direction with cloths. The rubbing process serves to establish a unique optical direction. After rubbing both plates, they are rotated from 90 to 270 degrees with respect to each other; adhered together using organic adhesives to preserve a constant thickness to a space or gap between the plates; filled with various mixtures of LC substances; and finally sealed using organic adhesives. At this stage, polarizing films are often attached by a lamination process. Finally, electrical connections are made to both plates in a manner consistent with the electrical and display designs.
The use of rubbed polymer films (alignment direction and tilt angle controlling films) dominates the process technology used in the production of all categories of nematic liquid crystal displays, and polyimides are the most common alignment films in use today. Moreover, the polyimide tilt angle and its magnitude are very important in the various electro-optic responses and the electro-optic properties of the LCD device. The stability, legibility and reliability of the LCD are all related to the magnitude of the tilt angle and to the unchanging nature of this magnitude.
Polyimide films used to control the alignment direction and the tilt angle of liquid crystal molecules in all types of liquid crystal displays are very thin, being on the order of 100 to 2000 angstroms. The tilt is induced in a unique direction of the polyimide polymer by gentle buffing with specific cloths. Twisted nematic (TN) LCD's, including active matrix (AM) TN LCD's, such as those used in pocket TV sets and watches, require lower tilt angles in the range of 3 to 5 degrees. Supertwisted nematic (STN) LCD's require higher tilt angles, typically between 5 to 30 and particularly between 5 to 15 degrees. The actual tilt angle obtained is a function of polymer ordering on the surface, the resulting surface energy, the nature of the cloth used to buff the surface and the amount of buffing pressure. In addition to these variables, each of the hundreds of commercial liquid crystal formulations interacts differently with a given surface. In general, however, the single most important factor determining the value range of the tilt angle is the intrinsic character of the polyimide used to control this angle.
Thus, polyimide alignment films for liquid crystal displays must exhibit certain key properties including stable and predictable alignment of liquid crystal molecules and moderate to high tilt angle. In addition, for active matrix displays, the polyimide alignment film must also have a high value of the so-called voltage holding ratio (VHR). The active matrix electrode layer comprises nonlinear addressing elements such as, for example, thin film transistors (TFT), metal-insulator-metal (MIM) diodes or metal-silicon nitride-indium tin oxide (MSI) diodes which are integrated with the image point. Each image point represents a capacitive load with respect to the particular active nonlinear element, which is charged at the rhythm of the addressing cycle. In this cycle, it is of paramount importance that the voltage applied to an addressed image point drops only slightly until the image point is again charged in the next addressing cycle. A quantitative measure of the drop in voltage applied to an image point is the voltage holding ratio (VHR) which is defined as the ratio of the drop in voltage across an image point in the nonaddressed state to the voltage applied. A process for determining the VHR is given, for example, by B. Rieger et al., in Conference Proceeding der Freiburger Arbeitstagung Flussigkristalle (Freiburg Symposium on Liquid Crystals), Freiburg, 1989. Electro-optical systems having a low or relatively low VHR show insufficient contrast.
Currently, aromatic fluorinated polyimides having moderate to high tilt angles are known for use as alignment layers in conventional twisted nematic and more advanced super twisted nematic displays. For example, Japanese Kokai Patent No. 63(1988)-259515, published Oct. 26, 1988, Japanese Kokai Patent No. 1(1989)-180518, published Jul. 18, 1989, and Japanese Kokai Patent No. 1(1989)-180519, published Jul. 18, 1989, disclose LCD device orienting agents, which consist of polyimide resins containing perfluoroalkyl groups containing from 1 to 6 perfluorinated carbon atoms.
Japanese Kokai Patent No. 62(1987)-127827, published Jun. 10, 1987 and Japanese Kokai Patent No. 62(1987)-87939, published Apr. 22, 1987, disclose compositions for liquid crystal oriented films containing a poly(amic acid) or polyimide and at least one of the tetracarboxylic acid or diamine components contains fluorine atoms.
Japanese Kokai Patent No. 2(1990)-4225, published Jan. 9, 1990, discloses a process for preparing a liquid crystal display element comprising heat treating a liquid crystalline orientation film of a polyimide obtained by treating an aromatic tetracarboxylic acid dianhydride with a fluorine-containing diamine in contact with liquid crystals to a temperature at least as high as the liquid transition temperature of said liquid crystal.
For active matrix applications, however, such fluorinated aromatic polyimides suffer from low voltage holding ratio (VHR). The polyimide alignment films of the present invention are based on a unique dianhydride, e.g. ethylenediamine tetraacetic acid dianhydride, and overcome the drawbacks of low VHR of the conventional aromatic polyimide alignment films. The polyimide films of the present invention yield very high voltage holding ratios when used as alignment films for liquid crystal display devices. They also show the ability, when modified with certain comonomers, to give high tilt angles, which are necessary for advanced applications in both active matrix displays (AMD) and super twisted nematic displays (STN).